Shark tracking and fisheries-independent assessments

Satellite tracking has, quite literally, opened up a new world for shark scientists, enabling them to achieve independence from data supplied by fisheries – data whose value, for a number of reasons, is limited.

Words by Jeremy Vaudo

Cover image

Photo by Brian Skerry | National Geographic Creative

Where do sharks go and when? What areas do they use? These are important questions for understanding the role of sharks in ecosystems because the answers will tell us when and where sharks are going to influence their prey populations. Answering these questions is also important to the conservation and management of sharks because it allows for the designation of critical habitats, such as nursery grounds, that can be protected.

But how do we figure out where the sharks are and where they go? It turns out fishers can be very helpful in this respect. Let’s face it, fishers on the whole, whether commercial or recreational, are very good at catching fish. In fact, fishers often know the hotspots for a given species before biologists do, and we biologists often use this information to pick our study sites and species. So by working with fishers, we can learn where, when and how often they catch sharks, and then design appropriate studies to research the movements of the sharks and their encounters with fisheries. Talk to enough fishers over a large enough area and get enough conventional identification tag recaptures and there’s a good chance that you can piece together a general migration picture. For example, in shortfin mako sharks in the western Atlantic Ocean, we see a northward progression of fisheries captures from the mid-Atlantic Bight in the USA to the Grand Banks off Canada as the seasons change from spring to autumn. Presumably, the sharks are following the warm waters north.

Data of this type are called fishery-dependent data because they are collected directly from commercial and recreational fisheries. Although they are extremely useful to biologists, they do have some drawbacks. Not only are we are at the mercy of the fisheries, but data collected in this manner are not standardised. Many fisheries are not targeting sharks, so depending on their target and local regulations, the gear, bait types and time of year spent fishing can differ between fisheries. The fishing locations also reflect the best places to catch the target species, not necessarily where the sharks we are studying are. So when it comes to shark movements and habitat use, fishery-dependent data can tell us that sharks are in an area where they are being caught, but the lack of capture information for an area during a given time of year doesn’t necessarily mean the sharks are not there. The lack of captures for an area could be because no one was fishing there. And even if sharks are being captured in a certain area, that may not be the best place to catch them for studies.

Although fishery-dependent data are certainly useful and can provide broad insights into potential fish movements, the biases that come with them make it difficult to answer fully questions about the details of shark movements and habitat use. What we need are additional sources of data that are not dependent on fisheries – we need, in fact, fisheries-independent data. One way for scientists to collect such data directly is to conduct fishing surveys themselves in a standardised scientific format. But because of all the fishing effort needed, these methods can be very labour intensive and therefore costly. And it is still possible that sharks, being highly mobile, may leave the survey area.

So how can we figure out where wide-ranging shark species are going in a way that is independent of fisheries and yet also provides the detailed information we need? If only the sharks could tell us! But it turns out that they can – by using a technique called satellite telemetry. This method makes use of specialised tags that can ‘talk’ to satellites, essentially saying, ‘Hey, I’m over here!’ The location of the shark can then be relayed back to researchers sitting in front of their computers in their comfy offices. It’s just like a tracking device you’d see in a spy movie. After the shark has been tagged, it will tell us where it is for as long as the tag’s battery lasts; we don’t have to recapture the shark, so we have achieved fisheries-independence!

Cover image

Photo by Brian Skerry | National Geographic Creative

At the Save Our Seas Shark Research Center in Florida, USA, in collaboration with the Guy Harvey Research Institute, we’ve been using satellite telemetry to study the movements and habitat use of several highly mobile shark species. As a result, we’ve observed a lot of really neat behaviour that could not be perceived using fisheries-dependent methods. We have learnt that some tiger sharks, which were thought to be a coastal species, actually spend half the year swimming out in the open ocean. Why did it take so long for scientists to realise that this species, which is large and well known, spends so much time far out to sea? There were limited records of tiger sharks caught offshore in fisheries, but really it comes down to the fact that no one was looking for tiger sharks in the open ocean – which just goes to show the importance of fisheries-independent data.

But it was during our shortfin mako satellite telemetry project that we found a new use for our fishery-independent data. The study started off as expected: mako sharks were moving out into the international waters of the open ocean and ultimately passed through the management jurisdictions of 19 countries. But it wasn’t long before sharks started telling us they were on land – they had been caught. Overall, 12 of our 40 satellite-tagged mako sharks (30%) were caught. Mako shark meat is valuable and although these sharks are not typically targeted by commercial fisheries, they are kept when they are caught. We had come across a fishery-independent way of determining a very important piece of information critical for improving stock assessments: the fishery mortality rate for mako sharks. The bottom line is that with this satellite tracking approach, we could tell which of our sharks had been caught without depending on the fishers to let us know.

Estimating fishery mortality is a key part of fisheries management. It tells us how much of the stock is being caught, which we need to know to determine whether it is growing or shrinking. Up to now, these estimates were always based on fishery-dependent data, which are largely reliant on self-reporting by fishers. We now had an independent assessment of mako fishery mortality rates – and it was striking. We calculated that a mako shark had a 72% chance of not being harvested in a fishery per year. Additionally, our estimate of fishery mortality was about 10 times higher than the previous estimate from 2012, which was based on fishery-dependent data. This new estimate suggested that mako sharks, which currently lack international protection in the western Atlantic, are being overfished. Since the publication of our work earlier this year, a new fishery-dependent stock assessment conducted by the International Commission for the Conservation of Atlantic Tunas (ICCAT) has corroborated our fishery-independent assessment. Our study and this example demonstrate the usefulness of satellite telemetry for independently assessing fishery mortality, especially in shark fisheries where it is difficult to get accurate fishery-dependent data.

Project

SOSF Shark Research Center

The SOSF Shark Research Centre (SOSF-SRC) is located in Florida and was established at Nova Southeastern University in 2009 by directive of the founder of the Save Our Seas Foundation.

The centre focuses mainly on scientific research aimed at increasing knowledge to aid the conservation, management and understanding of sharks and rays worldwide.

A hallmark of the SOSF-SRC is that it specialises in taking integrative, multi-disciplinary approaches to research and conservation, which include combining high-tech genetics, genomics and field work to illuminate holistically aspects of shark and ray science that would be difficult to decipher using single-discipline approaches alone.

The SOSF-SRC also serves as an academic unit within Nova Southeastern University and as such its function includes the training of students from around the world in marine research and conservation. Although advanced scientific research is the main focus of the SOSF-SRC, our staff also undertake educational and outreach activities involving primary (US middle) and secondary (US high) school students.